1. Field of the Invention
The present invention relates to the housing and protection of integrated circuit chips. More specifically, the present invention relates to a leadframe structure that offers enhanced thermal dissipation for integrated circuit chips.
2. Description of the Related Art
Modern integrated circuits (ICs) require protection from self-heating effects to operate reliably. One of the methods by which ICs are cooled is heat conduction through the leadframe. The leadframe typically consists of a metallic leadframe paddle on which the IC is mounted and lead fingers for providing electrically conductive paths from the bonding pads of the IC to external elements, such as circuit boards.
An example of a typical leadframe is illustrated in FIG. 1. The IC 10 is typically attached to a leadframe paddle 12. Electrically conductive lead fingers 14 are electrically connected to the IC 10 with wire bonds 16 and serve to provide electrically conductive paths from the IC 10 to external elements (not shown). The leadframe is typically encapsulated in a moulding compound 18.
The typical heat conduction paths are along the leadframe paddle 12 and through the moulding compound 18, which typically has low thermal conductivity. The lead fingers 14 generally do not contribute significantly to the thermal dissipation because of the relatively low thermal conductivity paths between them and the paddle 12.
As IC fabrication technology advances, more power generating elements are incorporated into single ICs, creating a need for mounting packages with increased thermal dissipation properties. One prior leadframe design, described in U.S. Pat. No. 5,146,310 by Bayan et al., improves thermal dissipation by connecting the paddle to four thermally conductive pins and also increasing the surface area of the leadframe paddle. The paddle pins allow the paddle to be connected to external heat sinks, such as a ground plane on a circuit board. Although the Bayan leadframe design increases thermal dissipation over standard leadframes, the pin count is increased by up to four relative to a standard package. In order to incorporate the Bayan design into low pin count packages, such as 8-lead dual in-line plastic (DIP) or 8-lead small outline integrated circuit (SOIC) packages, the package dimensions must be increased by up to 50 percent to accommodate the additional paddle pins. As a result, these packages must be redesigned to accommodate the Bayan leadframe structure, and are not pin-compatible with the standard leadframe.
Another leadframe structure, described in Japanese Patent Application No. 2-310954 by Isao Sasahara, increases thermal dissipation by placing the inner ends of the lead fingers in close proximity to the leadframe paddle and branching the outer ends of the lead fingers so that a portion of each lead finger is attached to a thermally conductive sheet positioned on top of the IC package. Although the Sasahara structure improves thermal dissipation over standard leadframe structures, additional tooling is required to trim and form the lead fingers after the leadframe has been encapsulated in the moulding compound and additional manufacturing steps are required to apply the thermally conductive sheet to the top of the IC package.
A third leadframe structure, described in Japanese Patent Application No. 64-86545 by Nakamura et al., extends portions of the leadframe paddle into the space between the lead fingers to electrically shield the lead fingers from each other. A side benefit of this design is increased dissipation of heat into the moulding compound as a result of the increased surface area of the paddle. However, thermal conduction between the paddle and lead fingers is not greatly improved in this design because the paddle extensions are made narrow in order to extend them between each and every pair of lead fingers. Relatively little heat can flow into these extensions because a narrow paddle extension has a higher thermal resistance than a broad paddle extension. As a result, relatively little heat is available in the extensions to transfer into the leads.